Carbon Monoxide Poisoning - ALERT

Pulse Oximetry:

Here are two errors that are commonly seen in reviewing medical records and physician's reports. The second is the more dangerous to the patient than the first.

1) "The percent of the blood was 100% as measured by a pulse oximeter." Of course 100% saturation is physiologically impossible! There are many reasons for this. 97-98% saturation is possible, normal, and should be the levels reported. Pulse oximeters are inaccurate for several reasons. One has to do with the way they sense oxygen saturation, their calibration and zeroing, etc. In general they are designed to show gross abnormalities, not subtle differences. For exact % saturation values, arterial blood should be drawn and sent to the laboratory.

2) "The % saturation of the blood was 96, 97, 98% as given by a pulse oximeter, therefore NO COHb is present." Pulse oximeters are "blind" to CO because of the two wavelengths of light used to make measurements. Unfortunately, COHb is lumped in with oxyhemoglobin (O2Hb), thus producing incorrect readings. A person could have 15% COHb on board (or more), plus 80% O2Hb, but the pulse-Ox reading would still be 95% saturation. The unaware clinician then fails to consider CO poisoning in his/her differential diagnosis. If CO poisoning is even remotely suspected based on history, clinical work-up, or gut feeling, blood should be drawn and sent to the laboratory for COHb, % saturation, pH, etc.

Note: Instead of an arterial blood draw, venous blood can be drawn and is perfectly satisfactory to accurately show COHb, if present.

Lesson 1: Don't be fooled by your pulse-Ox. It has uses, but not in diagnosing CO poisoning.

Lesson 2: Maintain a high index of suspicion for CO poisoning in the ER and clinic.

Lesson 3: Even if the COHb is not greatly elevated, you may still have a case of CO poisoning.

References:
Vegfors, M., Lennmarken, C. (1991) Carboxyhaemoglobinaemia and pulse oximetry. Brit. J. Anaesthesia Inten. Care, 66, 625-626.ABSTRACT: We compared measurements obtained with a pulse oximeter (SpO2) against values obtained from a CO-oximeter in a patient with carbon monoxide poisoning. SpO2 was equal to the sum of the oxyhaemoglobin (HbO) and carboxyhaemoglobin (HbCO) values over the range of HbCO from 30 to 1%. This confirms the experimental findings that pulse oximeters measure HbCO as HbO. The patient was treated with oxygen (FlO2 = 50%) and recovered without any sequelae. Under these circumstances, the half-life of HbCO was approximately 2 h.

Buckley, R.G. et al. (1994) The pulse oximetry gap in carbon monoxide intoxication. Ann. Emerg. Med., 24, 252-255.ABBREVIATED ABSTRACT: Pulse oximetry has been reported to be falsely elevated in the presence of carbon monoxide (CO). However, the degree to which pulse oximetry overestimates measured oxyhemoglobin saturation (O2Hb) has not been investigated in patients with CO exposure. This study quantifies the effect of CO on pulse oximetry and O2Hb in a series of patients with elevated carboxyhemoglobin (COHb) levels. Oxygen saturation as measured by pulse oximetry failed to decrease to less than 96% despite COHb levels as high as 44%. Regression between the pulse oximetry gap and COHb suggests that pulse oximetry overestimates O2Hb by the amount of COHb present. Pulse oximetry is unreliable in estimating O2Hb saturation in CO-exposed patients and should be interpreted with caution when used to estimate oxygen saturation in smokers.